Missing .twig files in wordpress theme editor

The thing you need to do is update the filter with the “allowed” editor file types.

Here’s what you’ll need for TWIG:

function add_custom_editor_file_types( $types ) {
    $types[] = 'twig';
    return $types;
}
add_filter( 'wp_theme_editor_filetypes', 'add_custom_editor_file_types' );

Of course, you can add any file type/extension you wish in the same manner. The editor UI will show the directory structure only where there are “editable” files.

EDIT:
Certain directories are excluded from being scanned for files, by default. One of these is vendor. This is likely because in some instances, the vendor dir will be huge and so loading and processing for this will have a cost associated with it.

To prevent vendor dir from being excluded, you can use the filter dedicated for that purpose:

function unexclude_theme_editor_dirs( $exclusions ) {
    // default exclusions: 'CVS', 'node_modules', 'vendor', 'bower_components'
    $unExclude = [ 'vendor' ];
    return array_diff( $exclusions, $unExclude );
}
add_filter( 'theme_scandir_exclusions', 'unexclude_theme_editor_dirs' );